1. Field of the Invention
The present invention relates to a method of designing a semiconductor device. In particular, the present invention relates to a method of laying out interconnections in a semiconductor device.
2. Description of Related Art
CMP (Chemical Mechanical Polishing) is known as a technique for planarization employed in a production process of a semiconductor device. The CMP is used for planarizing a metal interconnection or for planarizing an interlayer insulating film when a multi-layer interconnection structure is formed.
For example, the CMP is employed when a low resistivity Cu (copper) interconnection is formed by a method called a damascene process. More specifically, after a trench is formed on an insulating film, the Cu is deposited into the trench and then excess Cu outside the trench is polished away by the CMP. Here, in a case when a width of the formed Cu interconnection is large, a phenomenon called “dishing” that the CMP planarized surface is formed in a dish shape occurs. A technique aimed at suppressing the dishing of the Cu interconnection is described in Japanese Laid-Open Patent Application JP-2000-68277. According to the technique, a wide metal interconnection is divided into a stripe pattern.
The CMP is also used for planarizing an interlayer insulating film when a multi-layer interconnection structure is formed. FIGS. 1A and 1B are cross-sectional views illustrating a general process of forming an interlayer insulating film. In FIG. 1A, metal interconnections 1 are nonuniformly distributed, and thus there are a dense region R1 and a sparse region R2. Interconnection density of the metal interconnection 1 in the dense region R1 is higher than that in the sparse region R2. An interlayer insulating film 2 is so deposited as to cover the metal interconnections 1, by a plasma CVD (Chemical Vapor Deposition) method or a high-density plasma CVD (HDP-CVD) method. A surface of the interlayer insulating film 2 thus formed reflects the underneath metal interconnections 1 and is not flat. Specifically, the interlayer insulating film 2 in the dense region R1 is formed relatively thick, while the interlayer insulating film 2 in the sparse region R2 is formed relatively thin.
If the CMP is performed under the condition shown in FIG. 1A, a surface of the interlayer insulating film 2 in the sparse region R2 becomes lower than a surface of the interlayer insulating film 2 in the dense region R1, as shown in FIG. 1B. That is to say, although the flatness is improved locally, a global unevenness (dishing) remains on the surface of the interlayer insulating film 2 after the CMP. As a result, out of focus occurs in a photolithography process, which deteriorates reliability of the semiconductor device to be manufactured. Moreover, since the interlayer insulating film 2 in the sparse region R2 becomes thinner than that in the dense region R1, capacitance of the interlayer insulating film 2 varies spatially. Therefore, electrical characteristics of the manufactured semiconductor device are deteriorated.
Techniques aimed at suppressing the dishing of the interlayer insulating film are disclosed in, for example, Japanese Laid-Open Patent Application JP-H11-126822 and Japanese Laid-Open Patent Application JP-2002-342399. As described above, the dishing of the interlayer insulating film is likely to occur when variation in the interconnection density is conspicuous. Therefore, according to the techniques described in those patent documents, a dummy interconnection different from a true interconnection is formed in a region where the dishing may occur.
FIGS. 2A and 2B are cross-sectional views illustrating an example where interconnections including the dummy interconnection are formed. In FIGS. 2A and 2B, a region R1 corresponds to a region where interconnection density of the true interconnection is relatively high, while a region R2 corresponds to a region where interconnection density of the true interconnection is relatively low. As shown in FIG. 2A, a barrier metal film 4 is formed on an insulating film 3, and an Al film 5 is formed on the barrier metal film 4. The barrier metal film 4, which is for example a TiN film, plays a role of improving adhesiveness between the Al film 5 and the insulating film 3 and preventing the Al from diffusing into the insulating film 3. Further, a resist mask 6 having a predetermined pattern is formed on the Al film 5.
Next, a dry etching such as an RIE (Reactive Ion Etching) is performed by using the resist mask 6. As a result, as shown in FIG. 2B, true interconnections 7 are formed in the region R1, and dummy interconnections 8 in addition to true interconnections 7 are formed in the region R2. Only the true interconnections 7 of them are used for a circuit operation. In this manner, according to the conventional technique, the dummy interconnections 8 different from an interconnection pattern are formed in the region R2 where the dishing may occur. Consequently, the variation in the interconnection density is reduced and hence the dishing of the interlayer insulating film to be formed is suppressed.
The inventors of the present application have recognized the following points. To form the dummy interconnections 8 as described above means that the interconnection density as a whole is increased. If the interconnection density is increased too much, the following problem arises in the dry etching process shown in FIG. 2B.
In general, an end point of the dry etching is determined by monitoring emission intensity of reaction product generated during the etching. FIG. 3 shows transition of plasma emission intensity during the dry etching process. In FIG. 3, a solid line (Sample-A) represents the emission intensity in a case where the dummy interconnection 8 is not formed. On the other hand, a dashed line (Sample-B) represents the emission intensity in a case where the dummy interconnection 8 is formed, namely, the interconnection density is increased.
As the etching progresses, the emission intensity associated with the Al film 5 decreases, whereas the emission intensity associated with the barrier metal film 4 increases. When the emission intensity associated with the barrier metal film 4 is expressed as a negative value, the emission intensity as a whole begins to decrease during the etching, as shown in FIG. 3. A point at which the decrease stops is recognized as the end point of the dry etching. In the case of the solid line (Sample-A), the gradient of the decrease is large and thus the end point of the dry etching can be detected well.
On the other hand, in the case of the dashed line (Sample-B), an area occupied by the resist mask 6 becomes large because the dummy interconnections 8 are formed and thus an exposed area of the Al film 5 or the barrier metal film 4 becomes small. Therefore, the emission intensity associated with the Al film 5 and the emission intensity associated with the barrier metal film 4 become weaker than in the case of the above-mentioned Sample-A. Consequently, the gradient of the decrease of the emission intensity becomes smaller and thus the end point of the dry etching may not be detected precisely. If the end point of the dry etching is not detected precisely, over-etching occurs or etching residue remains. This causes deterioration of the electrical characteristics and the reliability of the semiconductor device.
As described above, when the dummy interconnections 8 are formed in order to suppress the variation in the interconnection density, the interconnection density as a whole is further increased and thus the problem arises in the dry etching process. Moreover, when the dummy interconnections 8 are formed, such a problem that interconnect capacitance is increased also arises.